Caisson (engineering) القيسونات

Caisson (engineering)

In geotechnical engineering, a caisson is a retaining, watertight structure used, for example, to work on the foundations of a bridge pier, for the construction of a concrete dam, or for the repair of ships. These are constructed such that the water can be pumped out, keeping the working environment dry. When piers are to be built using an open caisson and it is not practical to reach suitable soil, friction pilings may be driven to form a suitable sub-foundation. These piles are connected by a foundation pad upon which the column pier is erected.


Shallow caissons may be open to the air, whereas pneumatic caissons, which penetrate soft mud, are sealed at the top and filled with compressed air to keep water and mud out at depth. An airlock allows access to the chamber. Workers move mud and rock debris (called muck) from the edge of the workspace to a water filled pit, connected by a tube (called the muck tube) to the surface. A crane at the surface removes the soil with a clamshell bucket. The water pressure in the tube balances the air pressure, with excess air escaping up the muck tube. The pressurized air flow must be constant to ensure regular air changes for the workers and the height of the water in the muck tube must be carefully regulated to prevent unnecessary overpressure or low pressure which could allow excessive inflow of mud or water at the base of the caisson.
The caisson will be brought down through soft mud until a suitable foundation material is encountered. While bedrock is preferred, a stable, hard mud is sometimes used when bedrock is too deep.
Caisson disease is so named since it appeared in construction workers when they left the compressed atmosphere of the caisson and rapidly reentered normal (decompressed) atmospheric conditions. It is caused by the same processes as decompression sickness in divers. The Brooklyn Bridge was built with the help of caissons, and several workers were either killed or permanently injured by caisson disease during its construction, including the designer's son, Washington RoeblingCaissons have also been used in the installation of hydraulic elevators where a single-stage ram is installed below the ground level.

PILE TESTS

Pile tests are necessary to:
determine the expected settlement of the pile at working load or at some multiple thereof;
determine the ultimate bearing capacity of the pile; and/or
check the pile integrity (structural soundness).
In this unit, we will illustrate and describe some of the methods of testing piles. The tests described include both the static and the dynamic methods.
Methods of Testing Piles
There are many methods of testing piles. These methods can be grouped into three categories:
the static load principle,
the dynamic load principle, and
other tests.
For tests based on the static load principle, the load applied to the test pile is static, that is, not moving. Two types of tests based on the static load principle are:
the maintained load test (ML), and
the constant rate of penetration (CRP) test.
For tests based on the dynamic load principle, the load applied is dynamic, that is, moving or vibrating. Three types of tests based on the dynamic load principle are:
the driving formula test,
the Case-Goble test, and
the steady state dynamic response test.
Other tests include:
the sonic test, and
the coring test.

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What is the effect of high concrete temperature

What is the effect of high concrete temperature (above 77°F) on transporting, placing, and finishing concrete, and on hardened concrete properties?

A: When the temperature of freshly mixed concrete exceeds 77°F (25°C) there may be a number of effects on the fresh and hardened properties of concrete.One of the effects on fresh concrete properties is accelerated setting, which leads to a shorter time period for transporting the concrete to the job site, and a shorter window for placement, consolidation, and finishing of the material.
The higher temperature also leads to a higher water demand to maintain the concrete at the specified slump, which can tempt the contractor to add water to retemper the mixture leading to lower than expected compressive strength of the hardened material. In addition mixtures at high temperatures may require higher dosages of air-entraining admixtures to produce the required air content for durability in severe climates. The effect on hardened concrete properties are a high early strength but low ultimate strength compared to mixtures placed with a lower temperature. In addition, if no precautions are taken, there is an increased potential for plastic shrinkage cracking during the finishing operations, and increased potential for cracking due to volume changes caused by drying shrinkage and thermal effects.

Q: What precautions do I have to take during hot-weather concreting?

What precautions do I have to take during hot-weather concreting?

I am planning to have a large volume of concrete placed as part of an industrial development project that will be built during the summer of this year. I have been told that the quality of concrete can be affected not only by cold weather but also hot weather conditions. What are the effects of hot weather on concrete, and what precautions should be taken to assure that the concrete will provide good durable service?
A: It is true that hot weather conditions above approximately 25°C (77°F) can adversely impact the quality of concrete. The precautions that should be taken to assure a quality end product will vary depending on the actual conditions during concrete placement and the specific application for which the concrete will be used. In general if the temperature at the time of concrete placement will exceed 25°C (77°F) a plan should be developed to negate the effects of high temperatures. The precautions may include some or all of the following:
1) Moisten subgrade, steel reinforcement, and form work prior to concrete placement.
2) Erect temporary wind breaks to limit wind velocities and sunshades to reduce concrete surface temperatures.
3) Cool aggregates and mixing water added to the concrete mixture to reduce its initial temperature.
4) Use a concrete consistency that allows rapid placement and consolidation.
5) Protect the concrete surface during placement with plastic sheeting or evaporation retarders to maintain the initial moisture in the concrete mixture.
6) Provide sufficient labor to minimize the time required to place and finish the concrete, as hot weather conditions substantially shorted the times to initial and final set.
7) Consider fogging the area above the concrete placement to raise the relative humidity and satisfy moisture demand of the ambient air.
8) Provide appropriate curing methods as soon as possible after the concrete finishing processes have been completed.
9) In extreme conditions consider adjusting the time of concrete placement to take advantage of cooler temperatures, such as early morning or night time placement.With proper planning and execution concrete can be successfully placed and finished to produce high quality durable concrete at temperatures of 35°C (95°F) or more.

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